This invention relates to cubic boron nitride (CBN) abrasive compacts.
Boron nitride exists typically in three crystalline forms, namely cubic boron nitride (CBN), hexagonal boron nitride (hBN) and wurtzitic cubic boron nitride (wBN). Cubic boron nitride is a hard zinc blend form of boron nitride that has a similar structure to that of diamond. In the CBN structure, the bonds that form between the atoms are strong, mainly covalent tetrahedral bonds.
CBN has wide commercial application in machining tools and the like. It may be used as an abrasive particle in grinding wheels, cutting tools and the like or bonded to a tool body to form a tool insert using conventional electroplating techniques.
CBN may also be used in bonded form as a CBN compact, also known as PCBN (polycrystalline CBN). CBN compacts comprise sintered masses of CBN particles. When the CBN content is at least 70 volume % of the compact, there is a considerable amount of CBN-to-CBN contact. When the CBN content is lower, e.g. in the region of 40 to 60 volume % of the compact, then the extent of direct CBN-to-CBN contact is limited.
CBN compacts will generally also contain a binder which is essentially ceramic in nature. When the CBN content of the compact is less than 70 volume %, the matrix phase, i.e. the non-CBN phase, will typically also comprise an additional or secondary hard phase, which is usually also ceramic in nature. Examples of suitable ceramic hard phases are carbides, nitrides, borides and carbonitrides of a Group 4, 5 or 6 (according to the new IUPAC format) transition metal aluminium oxide and mixtures thereof. The matrix phase constitutes all the ingredients in the composition excluding CBN.
CBN compacts tend to have good abrasive wear resistance, are thermally stable, have a high thermal conductivity, good impact resistance and have a low coefficient of friction when in sliding contact with a workpiece. The CBN compact, with or without a substrate (the substrate having been integrally bonded to the PCBN layer during the sintering process) is often cut into the desired size and/or shape of the particular cutting or drilling tool to be used and then mounted on to a tool body utilising brazing techniques.
CBN compacts may be mechanically fixed directly to a tool body in the formation of a tool insert or tool. However, for many applications it is preferable that the compact is bonded to a substrate/support material, forming a supported compact structure, and then the supported compact structure is mechanically fixed to a tool body. The substrate/support material is typically a cemented metal carbide that is bonded together with a binder such as cobalt, nickel, iron or a mixture or alloy thereof. The metal carbide particles may comprise tungsten, titanium or tantalum carbide particles or a mixture thereof.
A known method for manufacturing the polycrystalline CBN compacts and supported compact structures involves subjecting an unsintered mass of CBN particles together with a powdered matrix phase, to high temperature and high pressure (HpHT) conditions, i.e. conditions at which the CBN is crystallographically or thermodynamically stable, for a suitable time period.
Typical conditions of high temperature and pressure which are used are temperatures in the region of 1100° C. or higher and pressures of the order of 2 GPa or higher. The time period for maintaining these conditions is typically about 3 to 120 minutes.
CBN compacts with CBN content of at least 70 volume % are known as high CBN PCBN materials. They are employed widely in the manufacture of cutting tools for machining of grey cast irons, white cast irons, powder metallurgy steels, tool steels and high manganese steels. In addition to the conditions of use, such as cutting speed, feed and depth of cut, the performance of the PCBN tool is generally known to be dependent on the geometry of the workpiece and in particular, whether the tool is constantly engaged in the workpiece for prolonged periods of time, known in the art as “continuous cutting”, or whether the tool engages the workpiece in an intermittent manner, generally known in the art as “interrupted cutting”.
Typically high CBN PCBN materials are used in roughing and finishing operations of grey cast irons, white cast irons, high manganese steels and powder metallurgy steels.
After extensive research in this field it was discovered that these different modes of cutting, machining operations and different type of workpiece materials place very different demands on the PCBN material comprising the cutting edge of the tool. Typically a PCBN material for high performance in these application areas should have high abrasive wear resistance, high impact resistance, high thermal conductivity, good crater wear resistance and high heat resistance, i.e. able to maintain these properties at high temperatures. The cutting tool tip can reach temperatures around 1100° C. during machining.
The combination of properties that provide for the above-mentioned behaviours in application can only be achieved by a material that has a high CBN content, higher than 70 volume % and a binder phase that will form a high strength bond with CBN, high toughness and that will retain its properties at high temperatures.
A conventional PCBN material design approach for high CBN content PCBN materials has been to use metal-based starting materials to react with the CBN and to form stable ceramic compounds as the binder phase. The high pressure and high temperature sintered PCBN material is practically pore-free and is ceramic in nature. Ceramic materials are known to have high abrasive wear resistance, high thermal conductivity, good crater wear resistance but they lack impact resistance as a result of their inherent brittleness.
The main problem is that the tools tend to fail catastrophically by fracturing or chipping mainly due to weakness in the binder phase, exacerbated by an increasing demand in the market for higher productivity. This typically results in a reduced life of the tool which necessitates regular replacement of the tool. This in turn, typically results in an increase in production costs, which is undesirable.
CBN is the most critical component of the high CBN content PCBN materials. It provides hardness, strength, toughness, high thermal conductivity, high abrasion resistance and low friction in sliding contact with iron bearing materials. The main function of the binder phase is therefore to provide high strength bonding to the CBN grains in the structure and to complement CBN properties in the composite, particularly in compensating for the brittleness of the CBN phase.
It is desirable to develop improved CBN-based materials that function more efficiently e.g. that exhibit improved abrasive wear resistance, thermal conductivity, impact resistance and heat resistance.